Data storage in the computer industry is accomplished in a number of ways. For example, data may be stored on various data storage media devices such as tapes, compact disks, "floppy" or "hard" disks, and the like. Oftentimes, data storage media which is transferrable from one location to another is housed within a parallelepiped-shaped cartridge. It is to be understood that the term "cartridge" or "data cartridge" as used in the present application encompasses any data storage media device, whether or not it is housed within a cartridge.
Data storage systems are used to store data cartridges at known locations and to retrieve desired cartridges so that data may be written to or read from the cartridges. A typical data storage system may include different types of cartridge receiving devices. For example, one type of cartridge receiving device is a cartridge storage rack or "magazine" which has a plurality of individual cartridge storage locations. Another type of cartridge receiving device is a cartridge read/write device or "drive". A data storage system may also include a cartridge handling assembly for retrieving data cartridges and translating them among cartridge receiving devices (e.g., from a cartridge storage location to a drive and vice-versa).
Data storage systems may be produced in a variety of sizes and configurations. One type of data storage system has a guide track extending along the length of a central vertical shaft. A single data cartridge handling assembly or "picker" assembly is vertically displaceable along the track and is adapted to move data cartridges between cartridge receiving devices positioned adjacent to the central vertical shaft in vertically stacked layers or tiers. In some storage systems of this type all components are provided in a single housing unit. In other storage systems of this type a plurality of modular housing units are stacked one on top of the other to provide a compound housing assembly. In a compound housing assembly each modular housing unit contains a portion of the vertical shaft and one or more of the tiers of cartridge receiving devices of the data storage system. Such an assembly of modular units is disclosed in U.S. patent application Ser. No. 09/137,350 filed Aug. 20, 1998 for MODULAR DATA STORAGE SYSTEM of Joseph M. White, Matthias Lester, and Dave Jones abandoned, which is hereby incorporated for all that it discloses. An advantage of an integral data storage system having multiple vertically stacked tiers accessible by a single picker assembly over systems having multiple independent single tier units, each with its own picker assembly, is that it obviates the need to coordinate the operation of multiple systems. Also, unnecessary duplication of certain system components, such as drives and pickers, is avoided. However one disadvantage of a unitary system having a single picker assembly is that a failure of the picker assembly causes loss of access to all of the media in the storage system until the picker is repaired or replaced. It would be generally desirable to provide a data storage system having multiple vertically stacked tiers accessible by a single picker assembly with a subsystem which would automatically replace a nonoperating picker assembly with an operating picker assembly so that the user's access to data stored in the system is not interrupted.